Strength of bolt-fixings in laminated strengthened glass

Kent PerssonDiv. of Structural Mechanics, LTH

Lund University

Glass in Building Structures • Many advantages from an architect’s

point-of-view, light, airy constructions

• More common to use glass as load bearing parts of the building structure

• More advanced structures requires more advanced material knowledge

• Lack of design criteria makes it difficult to design safe and secure buildings

Tensile Strength of Toughened Glass

• Floatglass– Ultimate strength bending: ~ 60 MPa– Design code value: ~ 30 MPa

• Toughened glass– Ultimate strength bending : ~ 200-250 MPa

(lower around holes and along edges)– Design code value: ~ 60 MPa

Bolted Connections

• Bolts used as joints to connect glass to glass and to other materials

• Methods to predict the mechanical behaviour of glass-bolt joints are required

Studies of Bolted Connections

• Establish relationships between loads and strength of the glass-bolt connection– Experimental tests and numerical analyses

• Utilize obtained relationships in design tool for point fixed glass

Design tool for glass - ClearSight

A computer based design tool, for analysing strengthof bolt-fixed laminated strengthened glass

Masters thesis by J. Malmborg

Design Tool for Glass contd.

• Easy-to-use computer program – the user must not be acquainted with the numerical method being used

• The result should be easy to interpret

• Program follows building design codes

• Based on the results, the user will be able to determine if the tested configuration holds

• Initially, one type of bolt and a few load cases

Geometry and Load Types

• Two different loads are considered: line load and distributed load

• Arbitrary number of bolts • Rectangular glass panes

Material Properties

• Material properties may be given for both glass and the intervening PVB-foil

• Stiffness properties, density and the design strength value of glass may given

• Default values of the parameters are coded into the program

Experimental Studies of Two Types of Bolts

Cylindrical bolt Countersunk bolt

In part from masters thesis by C. Bength

Experimental Test Setup

Strain gauges glued on the glass at the tension side, close to the edge of hole

Cylindrical Bolt in Compression Loading

Countersunk Bolt in Compression Loading

Experimental Results for Cylindrical Bolt

Experimental Results for Countersunk Bolt- load on countersunk end

Results of Compression Test

Force, (N) Tensile stress at failure, (MPa)

Cylindrical bolt 4600 177

Countersunk boltLoad on countersunk end

5600 127

Countersunk boltLoad on flat end

7200 172

Experiment - Cylindrical bolt in bending

Experiment - Cylindrical bolt in bending

Results of Bending Test

Bending Moment, (Nm)

Tensile stress at failure, (MPa)

Cylindrical bolt 210 50

PHalf bolt and glass pane shown.

Only glass pane shown.Stress concentrations around the hole.

Finite Element Analyses of Experimental Tests

FE-analysis for Cylindrical Bolt

σmax=155MPa σmax=168MPa

FE-analyses for Countersunk Bolt-load on countersunk end

σmax=147MPaσmax=120MPa

FE-analysis, cylindrical bolt in bending

σmax=60MPa σmax=150MPa

Visualisation of Results

Example 1: Facade Glass

Holes to close to edge!

Example 1: Facade Glass

Holes moved, strength not exceeded.

Exempel 2: Balustrade Glass

With two bolt fixings only, the glass will not withstand the loads according to the design code.

Exempel 2: Balustrade Glass

With four bolts, no dangerous stresses develops when subjected to load according to the design code.

Conclusions

• Svårt att förutse härdningsspänningarnas storlek kring hål och kanter – stor säkerhetsfaktor måste användas.

• Komplicerade spänningstillstånd kring hål pga mjukt mellanliggande PVB-skikt.

• Numeriska beräkningar överensstämmer braexperimentella data.

• Hur fungerar limmade infästningar mekaniskt?

Thank You